1. Field of the Invention
This invention relates to an improved method for preparing resilient multi-layer composite sheets having very high bulk by intermittently bonding a shrinkable layer to a gatherable layer.
2. Description of Related Art
Nonwoven sheets with high bulk are known. Resilient versions, intended for cushion and insulation end uses, utilizing highly-crimped resilient fibers that are carded or air-laid, are generally known as “fiberfill”. Bulk, cushion, and resilience originate from the crimp and the elastic modulus of the fibers. Some fiberfill structures are air-laid with random fiber orientation. Others are carded and cross-lapped to provide balanced properties. Many are lightly needlepunched to improve strength. Most rely on inter-fiber bonding at the fiber crossover points to prevent disintegration from repeated loading and unloading. Inter-fiber bonding is achieved with spray binders, powder binders, or by intermixing low-melting fibers with higher melting matrix fibers. In some cases it is achieved by using multiple-component fibers comprising a low melting component and a high melting component, such as sheath-core fibers wherein the sheath comprises a polymer having a lower melting point than the core polymer. Such bonded prior art products have limited conformability and drape because of the extensive fiber inter-bonding that is required to stabilize the structures. The level of bulk achieved with prior art products is generally limited unless the product is not entangled or bonded. Such unbonded products, however, are not durable, because they tend to collapse (mat-down) under loading.
Improved cushion/insulation products utilizing spirally-crimpable side-by-side multiple-component filaments are also known in the art. These products usually have a basis weight well in excess of 4 oz/yd2, with the fibers inter-bonded at their cross-over points. For example, U.S. Pat. No. 3,595,731 to Davies et al. (Davies) describes side-by-side bicomponent fibrous materials containing bicomponent spirally crimped fibers which are bonded mechanically by the interlocking of the spirals of the crimped fibers and by melting the lower-melting fiber component, to achieve inter-fiber bonding at the crossover points. Crimp can be developed and the potentially adhesive component activated in a single step, or the crimp can be developed first, followed by activation of the adhesive component. Crimp is developed without applying pressure to the sheet, to allow the fibers to develop their full crimp-potential. Such products have high bulk, but do not have the extremely high bulk required in end uses such as pillows and the like, or the drapeability required in end uses such as apparel linings, because, as in products formed from planarly crimped fibers, the degree of bulk generated by fiber crimp alone is not sufficient for such end uses.
U.S. Pat. No. 5,382,400 to Pike et al. (Pike) describes a process for making a nonwoven fabric, which includes the steps of melt-spinning continuous multiple-component polymeric filaments, drawing the filaments, at least partially quenching the multiple-component filaments so that the filaments have latent helical (spiral) crimp, activating the latent helical crimp, and thereafter forming the crimped continuous multiple-component filaments into a nonwoven fabric. The resulting nonwoven fabric is described as being substantially stable and uniform and may have “high loft”, again all bulk or “loft” originating entirely from the fiber crimp.
U.S. Pat. No. 3,671,379 to Evans et al. (Evans) describes self-crimpable composite filaments which comprise a laterally eccentric assembly of at least two synthetic polyesters, the first of said two polyesters being partly crystalline, in which the chemical repeat-units of its crystalline region are in a non-extended stable conformation and the second of said two polyesters being partly crystalline in which the chemical repeat-units of the crystalline region are in a conformation more closely approaching the length of the conformation of its fully extended chemical repeat-units. The composite filaments are capable of developing a high degree of helical crimp against the restraint imposed by high thread count woven structures, which crimp potential is unusually well retained despite application of elongating stress and high temperature. The composite filaments increase, rather than decrease, in crimp potential when annealed under tension as a part of the fiber production process. The filaments are described as being useful in knitted, woven, and nonwoven fabrics, and may also be useful in forming bulky/resilient structures. High frequency crimp generation is described, and reference is made to high bulk, without quantification, all of the bulk being generated from fiber crimp.
Bulky elastic composite nonwoven materials are also known in the art. Examples of such materials include “stretch-bonded” and “neck-bonded” laminates. Stretch-bonded laminates are prepared by joining a gatherable layer to an elastic layer while the elastic layer is in an extended condition so that upon relaxing the layers, the gatherable layer is gathered. “Neck-bonded laminates” are produced by joining a necked, non-elastic layer with an elastic layer when the non-elastic layer is in an extended condition. The elastic layer in these products generally comprises an elastic film or an elastic nonwoven web such as an elastic meltblown web.
There remains a need for highly bulky, resilient and durable fibrous sheets that do not rely on extensive inter-fiber bonding or extensive inter-fiber entanglement to produce stable structures, that can withstand repeated loading and unloading without collapsing, and do not rely purely on the development of very high fiber crimp to produce resilient bulk.